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Civil Engineering Magazine THE MAGAZINE OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS

Birds in Flight Inspire Newest Copenhagen Bridge

By Catherine A. Cardno, Ph.D.

The movable pedestrian and cycle bridge in Copenhagen will curve in two planes as it extends across the city's inner harbor.

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The winner of a design competition to create a movable cycle and pedestrian bridge in Copenhagen, Denmark, is an elegant ribbon that curves in two planes to link misaligned streets on opposite sides of the harbor. © Wilkinson Eyre Architects

August 4, 2015—When it opens in 2017, the new location of the Dansk Arkitektur Center in Copenhagen, Denmark, will be a celebration of architecture and urban development in its structure as well as its exhibitions. It's only fitting, then, that the pedestrian and cycle bridge that will cross the city's inner harbor adjacent to the center's new location will do the same. With lines reminiscent of a bird in flight, the bridge will be an elegant ribbon that curves in two planes to link misaligned streets on opposite sides of the harbor, Vester Voldgade and Langebrogade. The bridge will also swing open, providing a 35 m navigation channel through the harbor.

The new bridge is slated to be built a stone's throw from the city's well-known Langebro bascule bridge. Only the third crossing in this section of the harbor, the dedicated pedestrian and bicycle bridge will relieve the mixed-use congestion of the Langebro bridge and provide direct access to the OMA-designed Bryghusgrunden project, which will house the architecture center. (See "Work Begins on Complex Copenhagen Venue Located over—and under—Busy Roadway," Civil Engineering , July/August 2013, page 24-25.)

The 180 m long steel bridge will be a curving white ribbon with a 4.5 percent gradient, rising from the quaysides to arc up to 5.4 m above the water. Last month, the design was selected as the winner of an international design competition run by Copenhagen-based Realdania, a philanthropic organization dedicated to improving the quality of life within Denmark through the built environment.

"The most defining feature of the bridge is its twisting 'wing' form at the deck edge, which starts as an outward-facing element at the banks, before twisting up to become an inward-facing element at mid-pan," says Simon Roberts, an associate in the London office of architecture firm WilkinsonEyre, one of the two architecture firms on the design team. 

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A twisting “wing” at the deck’s edge is an outward-facing beam element at the quaysides that becomes an inward-facing element at midspan. Formed from longitudinal, stiffened, triangular box girders, the wings offer open views and appropriate clearance over the navigation channel. © Wilkinson Eyre Architects

"One of the main concerns of the client was that our bridge shouldn't overshadow the Langebro bridge and shouldn't try and compete with it," says Roberts. "We ruled out any high-masted structures—cable-stayed or suspension bridges or anything like that—because we didn't want the appearance of the bridge to detract from the existing Langebro bridge."

The decision to twist the beam from an under-the-deck support at the quaysides to a position above the deck at the center of the harbor was made so that as much height as possible could be provided over the navigation channel, while simultaneously proving a gentle gradient change to meet the needs of pedestrians, bicyclists, and those using wheelchairs. The design also offers very open promenade views, according to Henning Stüben, a partner at the Copenhagen office of the architecture firm Urban Agency, the second architecture firm on the design team. (The London office of international engineering firm BuroHappold led the project team and completed the structural engineering for the bridge.)

Because the landing points of the bridge on the quaysides were predetermined, the design competition required innovative solutions for how to provide an adequate navigation channel while maintaining the necessary gradients. "You needed more length in order to get up to the navigation channel height and then down again in a universally accessible way," explains Stüben. 

"On the first day that we sat together with WilkinsonEyre and BuroHappold, we …sketched the boomerang shape and we said this is the most rational way to do this, and the most elegant way," Stüben recalls. "And we were surprised that none of the other competitors in the first place had the same shape in plan."

"With it being a cycle bridge, we had to be very conscious of turning circles for cyclists," adds Roberts. "So that's why we have the sort of curves that we do, and we try and maintain a graceful arc that sweeps across the bridge in a continuous movement."

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The two central spans of the bridge will swing open when additional clearance is necessary within the navigation channel. The moveable bridge deck sections are fixed rigidly to pier arms that rotate to help maintain the bridge’s clean profile. © Wilkinson Eyre Architects

The five-span, stiffened-steel-deck bridge will contain two double-cantilevered, movable spans located atop its central piers, according to Davood Liaghat, CEng, Ph.D., Eur Ing, the group director for bridges and civil structures at the London office of BuroHappold and the lead consultant for the project. Liaghat wrote in response to written questions posed by Civil Engineering online. 

The bridge structure comprises two main longitudinal, stiffened, triangular box girders—the "wings"—-located on either side of the deck and connected transversely at intervals by V-shaped cross members. "The structural depth of the main girders varies significantly across the bridge," Liaghat noted. "The girders are deeper at the ends of the bridge and rotate upward towards the central position. This creates a simple yet intriguing aesthetic.

"The demand on the structure during the closed and open modes is the main challenge," Liaghat added. "This is an opening bridge and as with most of these type of bridges the devil is in the detail."

The majority of the electrical equipment that will operate the two moveable deck sections will be housed below deck level in one of the approach ramps, according to Michael Thorogood, CEng, a specialist on moveable structures and a director of the Rotherham, United Kingdom-based Eadon Consulting, who wrote in response to questions posed by Civil Engineering online. Eadon designed the operating equipment for the new bridge.

The movement mechanisms will be hidden within the piers. The two moveable bridge-deck sections will be fixed rigidly to rotational pier arms. Counterweights will be located in the tail end of each moving deck section.

Each of the moveable deck spans will be supported on a large-diameter slewing bearing located within its pier, according to Thorogood. "The internal race of the slewing bearings will be equipped with integral gear teeth," he explained. "The rotational equipment will be located inside the slew bearing and will consist of multiple electrical motors, each driving a pinion [that] engages with the internal gear teeth on the slewing bearing."

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The structural depth of the triangular box girders changes continuously along the bridge’s length, as the wings appear to rotate from a position under the deck to a position above it. © Wilkinson Eyre Architects

The use of hydraulic cylinders at the joint between the two moving spans will help to maintain the bridge's sleek silhouette. In the design, "four cylinders are arranged in two pairs, one pair at each edge of one of the moving spans," Thorogood explained. "One cylinder of each pair, located at handrail level, generates a compressive force at the joint. The other cylinder in each pair, located below deck level, generates a tensile force between the two moving decks. The compressive and tensile forces form a couple at the joint [and] it is this couple which allows [the] moment to be transferred between the two decks." 

Deck flaps formed with multiple plates to accommodate the curvatures of the deck sections will be located at the joints to ensure that there are no gaps in the bridge's walking and cycling surfaces, according to Thorogood. 

When the bridge is set to open, booms will swing out from a housing located between the cycle and pedestrian lanes to block the bridge's lanes from use. 

To minimize disruption to the harbor, streamline construction time, and guarantee the quality of the finish, the superstructure will be fabricated off-site with faceted steel plates that will be "welded together in a complex geometry to give it its unique, sleek appearance," Liaghat noted. The piers will be formed with precast, reinforced concrete that will also be transported to the site by ship. 

The bridge's piers are expected to be founded on piles, but an additional site investigation is required to formulate the final design. "As with most natural water bodies, we can expect soft alluvial deposits overlaying stronger geology," Liaghat noted. "It is envisaged that sheet piles will be installed around the foundation perimeter, the internal soft soil will then be dredged out and backfilled with sand. The internal water will then be drained to just below the foundation founding plane. The superstructure's precast elements will then be positioned and the sheet pile wall cut at low water level." 

The design of the abutments is also being refined. This includes both the manner in which the abutments will be anchored over the historical quayside wall as well as how the pedestrian steps and bicycle ramps will be detailed at either end of the bridge.

The expectation is that up to 12,000 pedestrian and cycle trips will be made across the bridge daily, and it is anticipated that the bridge will open approximately 200 times a year, according to Stüben.

Realdania is gifting the bridge to the city to support the development and accessibility of Copenhagen harbor, according to material distributed by WilkinsonEyre. The bridge is anticipated to open in 2018.

 

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